Homebrewed Electricity > Solar

Making decent solar panels part 1

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oztules:
It's something I said I would never do..... but now I have.



If you have no value on your time, and want to do your own DIY solar cells, then it has always been possible to cobble some cells together, stick them to a surface, and place plastic or glass over them, measure the output..... and generally feel good.

A fancy frame helps with the karma... but you know in the end it is only temporary.

As the water vapour creeps into the sealed wooden framed ones, or the window glass you scrounged up...breaks because of the heat/cold and localized heating in particular cells, and it all falls in a heap. It can get pretty sad for a lot of devotees that have tried.

There are plenty of youtube stories and how too's...... but all (but the ones using sylgard (or whatever it's called)) are doomed to failure)...... except those who have tried to use EVA (ethylene vinyl actetate).

I viewed a few treatments on the youtube sites, and none of them inspired a great deal of comfort. They all revolved around using a heat gun and a heap of good luck to melt the EVA around the cells, and onto the glass.........

Why I thought, did they make solar cells 30 years or more ago, and still basically the same today...and yet the amature panel maker comes up with lots of bad ways to do it??

I pondered this for a while, and in the meantime, a plumber friend turned up with a few solar hot water panels..... also  25 or more years old..... they were stuffed..... but they did use tempered glass, and it was iron free glass.... and it was free glass ( this was the clincher).

So I sat and looked at a few 2 sq meter panels (1x2m).......... yep, had to have a go too.... but it cant be cut!



This is where I decided to look around the world at the current state of play. This was sobering, no real effort at making proper panels like the big boys  built without the big boys price tags... why not?

It appears that folks thought you needed a laminator style of machine, which were very expensive, and a barrier to making your own. But if you look at what needs to be achieved, it is really only a few simple steps.... so lets look at the how and why.


Traditionally they use a sheet of glass (iron free tempered if you can get it), a sheet of EVA, the cells themselves, another sheet of EVA, and whatever your having yourself. After the last EVA sheet, you only really need to put something there that will not wick, and will give you something for the very very sticky EVA to bond to that is not your "jig" It can be painted later if you want more protection. Usually they use tedlar, but really any inert material will do for protection.

The simple theory of what we are trying to achieve is this:
We need to get the cells fully encapsulated in a UV stable envelope, that resists water and air from denaturing the cells. It must allow for differential expansion rates to occur between the silicon wafers, the glass and the medium itself....... and no bubbles.

Ethelyene vinyl acetate is a plastic sheet .5mm thick. It is soft plastic in texture and seems innocuous enough....... however, when it gets to only 65 degres C, it melts. This means that we need not get to very high temperatures to get something to happen.

I'll bet there are plenty of hopefuls ( like me for instance) that grabbed some EVA, some glass, and the wifes oven, and made a prototype cell......but it is filled with bubbles!..... It does stick to the glass and, it envelops the cells well, but it is useless because of the bubble population.

It is at this point  that you start to get a bit anxious about where to get a laminator...... but you don't need one. We just need to emulate what it does.

Next thing we look at is what happens when it melts. To get the EVA to be useful, we need to get enough energy into the stuff to cross link the molecules.  It changes the way the material behaves, and increases the melt point considerably as well. At low temp (65C-80C) it melts to itself very well, but is only slowly converted into the final product, so we have to keep it at these temperatures for long periods..... but as we increase temp, things happen a whole lot faster, and 5 mins may be enough at 145c to get it to cross link, and stick very strongly to the glass and the cell itself.

It also needs some encouragement from pressure..... or a vacuum perhaps, and use the air pressure to do the job of supplying the pressing force.

Now we know it  works perfectly well without vacuum, but you will have myriads of fine bubbles.... blocking out the sunlight.... not so perfect after all.

So we need a vacuum pump. Ideally one that can draw a near perfect vacuum. The vacuum you can achieve sort of directly relates to the bubbles you will get left with. High vacuum.... no bubbles. It's that simple. The vacuum will also press the cells very firmly (200 plus pounds per cell) against the glass, and this will help the glass bond, and make for a very flat cell, with no voids in the plastic envelope.............as always there is a but:... you must have NO leaks at all, or you will get uneven vacuum in the sheets, and you will get bubbles forming.... probably in the inter cell space, but bubbles all the same.

The heating profile I will use is like this:

15 mins at room temperature with full vacuum
15 mins at 65C with full vacuum
70-80 mins at 100C (or thereabouts with full vacuum

And if we can get the temp to rise some more.... then good. Even if the plastic garbage bag fails at this point, it's work is done for all practical purposes... better if it does not..but we get what we can.

So it looks doable.

First step for me was a BIG oven.

An oven that would handle the 2 meter by one meter panes of glass that came from out of the solar water panels. (it turns out that for some quirk of history, there are plenty of these old solahart type panels laying around over here).


We know that the temperatures involved are not high in order to achieve the bonding and cross linking that is required to get the cells to be protected, and pressed against the glass without any bubbles..... so I started with a wooden construction..... yes for an oven.

I had recently pulled down a wool shed for a fellow, and I was given the wood.... lots of old 4x2 hardwood.... so this was to be the frame
It looks like this:



I was also given some packing material in 8x4 foot sheets of low grade 3/4" pineboard.(someones kitchen apparently turned up in them). These would serve for the bottom and lid of the oven. For the sides I just used some of the old floor boards from the shearing stand.....

The whole thing is lined with ceiling roof bats insulation and then all held in the walls with a sheet of TYVEC building wrap material.... could be anything really.... even paper.



Ok, that gets us the oven walls and floor, and the lid was just a sheet of the 8x4 pineboard with insulation laying on the top.... Now we have an insulated container that will take our panel.





What this means to the 60watt panel builder.....is that even an insulated cardboard box will do for an oven (I used one with a fan heater blowing from in front for the initial trial, with blankets and dooners for the insulation...... perfectly good for a 60 watt panel)


Next step is a heating element.

This was simply solved by destroying a fan heater/radiator. These things draw about 2000 watts and incorporate a fan and a nichrome heating coil. They spill out a fair stream of not too hot air, so are unlikely to melt our plastic bags (comes later) when we bring the oven up to temperature.

I built the heater body out of a "think safety" sign (ironic really), which housed the heating element at one end, and the fan motor at the other. The fan blades were plastic, and would melt in this environment (proximity to the glowing element), so I stole the aluminium blades from my big MIG welder, and put the plastic ones back in that.

This gives us a 2000w heater that blows hot air..... but not too hot,and will allow us to gently heat the oven up to about 100-120 degrees, without any cunning trickery required to temp control it. Because the oven is so well insulated...... we just keep adding watts from the heater until we get to where we want.

Finished it looks like this:



Perhaps finished was too strong a word to use...... but it does work.



Thats about as much as we can do with this software in one hit, so ...

Thats Part one finished


............oztules

gsw999:
Hopefully by summer I will get some solar panels knocked up but I will be using old large double glazing panels, looking forward to seeing the finished article.

walp:
Amazing! :)
But it seems very advanced!

I have a couple of cells that need to be put togheter in a box.
If I use perspex\PMMA\Plexiglass at least the cover will not break, and if I make a sealed wooden box with built in ventilating fan, wouldnt that be enough?  
Unlike you Aussies, I live in Sweden, and we dont have super high temperatures or super humid air,  ::)



Keep us posted with the process!

Volvo farmer:
Nice write up!

As you progress, I'd like to see a cost breakdown of the cells/tabbing wire/ EVA and whatever else you had to buy.  I think it's great you are able to scrounge so much material so far. That's always been in the spirit of this site if you ask me  :)

oztules:
Walp,

I have no intention of dissuading you from doing as you have suggested. Many on the web have gone that way...... and are proud of their achievements

Sadly I see no future in it, and in the long run I think you would be best to buy proper panels instead if you intend on using them for power, rather than fun.

There is an enormous amount of time that goes into making the cells ready to mount in the first place. At that point you get to chose how long you wish to see your efforts produce effective power.

If is just for a giggle, to see how it works, and educate yourself, then it is all worth it, even if you only use sticky tape on the back of a sheet of glass to mount them. It is uplifting to see the sun turn into power.......

However, if you want the things to last 25 years or more, then you must look at it differently. There are only 2 ways I have discovered to allow us to get good results electrically, and mechanically over that time period. The Sylguard is a simple and effective way  (and darn expensive), but I don't have any results to point at to say how long this system has actually lasted. I suspect it is as good as the EVA approach..... but I have no proof.

Eva onto toughened glass has a track record, and most folks know of panels that are at least 1/4 of a century old and still running fine. The EVA of today is better than the EVA of yesteryear, and will be less likely to change colour than the older material. I have some BP panels 15 years old, which have clearly changed to a brownish tinge over the cells... from the heat, not the UV alone. The intercell spaces are still clear, as are the last 1/2 inch of the outer perimeters of the cells.... but even then, the performance is probably only 10% less than stated on the stickers.

If you build the panels for 50c per watt, and they only last a few years.... then it would have been cheaper to buy them made.... as they should have lasted at least 12 times longer.... so to keep replacing is $6.00/watt......??

You can still use your plastic with the EVA, just keep the temp low, and it will work just as well. Cooling is difficult with sealed box arrangements, and your output will reflect the fact that you will be building a solar heater..... with them inside where the water usually is (solar hot water ).


Volvo.

The oven was free, the tyvec was left over from a building project (house wrap). I put the call out for solar hot water panels... I now have had 20 delivered..... thats 4kw... and more coming I believe..... all free.... don't know what I'm going to do with them all....

So the glass is toughened and iron free, the panel frame is aluminium... comes with the glass, and all the rubber fittings etc. Just need to cut vent holes in the back for convective cooling... the only cost is the cells ( somewhere around 40c/watt from memory.... I bought 2kw of cells from the USA for $950 delivered to here.... they were supposed to be short tabbed, so the vendor gave me $150 bucks off if I kept them rather than returning and replacing them.... so about $800 for 2.2kw (he sent a heap extra). This included 1000 feet of tabbing wire and some 100 feet of bus wire..... so that was included, as were the flux pens.... Fredv8 or something similar on ebay also has tabbed ones for only 38c/watt for those folks in the USA. I got mine fromJeff at  MLSOLAR... good bloke.

The heater was free from the tip (dump), and the vacuum pump was from China (about $150 delivered) The EVA was from China, about $550 for 60 meters (enough for over 3kw).... about 19c/watt.
 If we amortize the pump over the 2kw we see about 7c/watt

Looks like this:
Cells          .40
EVA           .19
Pump        .07
Thus,  for cells we can expect to last the same as the commercial ones, it comes to around $0.66/watt.... complete with enclosures.


Without the defunct solar water panel, this would not be possible, (glass and aluminium) but the doors you used would fit the bill ok, and any kind of frame you felt like using to match..... and you would have gotten 2 panels instead of 1 per 2 pieces of glass in your case.


.............oztules

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